1. Field of the Invention
The present invention relates to level-detection circuits. More particularly, the present invention relates to Schmitt triggers having hysteresis.
2. Description of Related Art
Level-detection circuits having hysteresis, often referred to as Schmitt trigger circuits or simply Schmitt triggers, are commonly used in integrated circuits for receiving signals external to the integrated circuit. The Schmitt trigger is commonly used to reduce the impact of a noisy input signal on the integrated circuit. However, existing Schmitt triggers suffer from some problems. For example, in modern electronics, there is a growing desire for components with the capability to be powered off, such as portions of notebook computers or cellular phones. In those situations, it is important that the components be in the same state when powered back on as they were when powered down. For instance, a Schmitt trigger circuit may have a different trip point depending on whether the last state of the output signal was high or low. It would be desirable to have a Schmitt trigger that can be powered down, and when powered back up, demonstrates the same voltage transfer characteristics as the state the Schmitt trigger was in at power down.
Another problem with existing Schmitt triggers involves the difficulty of achieving precise control over the hysteresis due to process variations that may occur during manufacturing of the components used in the circuit. More specifically, the different trip points (i.e., 0-to-1 and 1-to-0) of a typical Schmitt trigger are generally controlled by appropriately sizing the devices (typically, both N-type and P-type transistors) used in the circuit such that a ratio of the on-resistance of one device to another device creates a feedback that modifies the trip point of the circuit differently in each direction. However, designing a specific amount of hysteresis is difficult because slight variations in processing those components introduces significant variations in the on-resistance ratio of the specific components from the design ratio to the actual as manufactured ratio. In addition, the effects of back-bias and source voltage of transistors on the circuit can be difficult to predict. Accordingly, there is a need in the art for a Schmitt trigger that is less susceptible to process variations and which can be powered down without losing the last state.
Briefly stated, the present invention enables a level-detection circuit, such as a Schmitt trigger, having hysteresis characteristics that are less susceptible to process variations and which can be powered down without losing the last state of the circuit. More specifically, the present invention makes use of detection circuitry to detect a level of an input signal, and to modify an intermediate output signal based on the level of the input signal. The circuitry also includes trip-level adjustment circuitry to alter the voltage at which the detection circuitry changes the intermediate output signal relative to the input signal. By switching the state of the output signal based on the intermediate output signal (as influenced by the trip-level adjustment circuitry), the voltage transfer characteristics of the entire circuit may be changed thereby introducing hysteresis. In addition, latching circuitry is included in the circuit to store a last state of the circuit in the event of a power down. In that way, the circuit may be powered up in the last state, thereby displaying the same hysteresis characteristics after the circuit is powered back up.
In one aspect, the invention makes possible an apparatus for generating an output signal based on an input signal. The apparatus includes a first detection circuit configured to detect a potential of the input signal and to provide an intermediate output signal. The intermediate output signal has a level, such as a voltage potential or current, based on the input signal. The apparatus further includes a second detection circuit coupled to the intermediate output signal and configured to provide an output signal based on the level of the intermediate output signal. More specifically, as the intermediate output signal passes through a threshold potential of the second detection circuit, the second detection circuit changes state. The apparatus also includes a trip-level adjustment circuit configured to provide feedback to the first detection circuit and thereby influence the input voltage at which the level of the intermediate output signal changes. In this configuration, the trip-level adjustment circuit can cause the intermediate output signal to pass through the threshold potential of the second detection circuit when the input signal is at some potential different from the threshold potential of the first detection circuit.
In another aspect, the invention makes possible an apparatus for generating an output signal based on an input signal and that includes power-down capability. The apparatus includes a first detection circuit, coupled to receive the input signal, that detects a potential of the input signal and that provides an intermediate output signal having a level based on the potential of the input signal. The first detection circuit is also coupled to receive an enable signal that deactivates the first detection circuit in response to the enable signal being deasserted. The apparatus also includes a second detection circuit coupled to receive the intermediate output signal and to switch the output signal when the level of the intermediate output signal passes through a threshold potential of the second detection circuit. A trip-level adjustment circuit is coupled to receive the intermediate output signal from the first detection circuit and is further configured to alter the input voltage at which the intermediate output signal changes. In that way, the trip-level adjustment circuit effectively alters the threshold potential of the first detection circuit thereby altering the voltage of the input signal at which the second detection circuit changes state (i.e., hysteresis is introduced). The circuit additionally includes a latching circuit coupled to receive the intermediate output signal and to receive the enable signal. The latching circuit is configured to latch a state of the intermediate output signal in response to the enable signal being deasserted. In that way, the last state of the circuit may be recreated when the circuit is again enabled.
In yet another aspect, the invention makes possible a method for powering down a circuit that generates an output signal based on an input signal and that remember the circuit""s last state at a power-down event. The method includes receiving an input signal at a level-detection circuit and generating an intermediate output signal by the level-detection circuit based on the input signal. In addition, the method includes receiving the input signal at a trip-level adjustment circuit and influencing the level-detection circuit by the trip-level adjustment circuit such that a level of the intermediate output signal changes when the input signal exceeds a first threshold, the first threshold being based upon a component in the trip-level adjustment circuit in combination with a component in the level-detection circuit. Moreover, the method includes receiving an enable signal indicating that a power-down event of the circuit is occurring. When the power-down event occurs, the intermediate output signal is in some last state. Thus, the method includes latching the last state of the intermediate output signal for the entire power-down period until a power-up event occurs.
Advantages of the present invention over existing level-detection circuits include that the hysteresis of the circuit is more easily estimated. More particularly, the hysteresis of the overall circuit can be simplified to the difference in trip points of two inverters having different transistor ratios. The trip points of inverters are easy to estimate using computer simulations, which is an improvement over existing level-detection circuits in which the hysteresis depends on various devices interacting with each other and which are not as easily computed.